This invention relates to novel polyimide gas separation membranes derived from alicyclic dianhydrides. This invention further relates to a process of separating gases using said membranes.
Membranes have been used to recover or isolate a variety of gases, including hydrogen, helium, oxygen, nitrogen, carbon monoxide, carbon dioxide, water vapor, hydrogen sulfide, ammonia, and light hydrocarbons. Applications of particular interest include the separation of hydrogen or helium from gas mixtures such as gas mixtures containing nitrogen, carbon monoxide, carbon dioxide, water vapor, and/or light hydrocarbons. For example, the separation and recycle of hydrogen is often necessary in various hydrocracker, hydrotreater, and catalytic cracking processes used in the oil refinery industry. Membranes can be used to achieve such separations.
Such membrane separations are based on the relative permeability of two or more gaseous components through the membrane. To separate a gas mixture into two portions, one richer and one leaner in at least one component, the mixture is brought into contact with one side of a semi-permeable membrane through which at least one of the gaseous components selectively permeates. A gaseous component which selectively permeates through the membrane passes through the membrane more rapidly than at least one other component of the gas mixture. The gas mixture is thereby separated into a stream which is enriched in the selectively permeating component or components and a stream which is depleted in the selectively permeating component or components. A relatively non-permeating component passes more slowly through the membrane than at least one other component of the gas mixture. An appropriate membrane material is chosen for the gas mixture so that some degree of separation of the gas mixture can be achieved.
Membranes for gas separation, particularly hydrogen separation, have been fabricated from a wide variety of polymeric materials, including cellulose esters, aromatic polyimides, polyaramides, polysulfones, polyethersulfones, polyesters, and polycarbonates. An ideal gas separation membrane is characterized by the ability to operate under high temperature and/or pressure while possessing a high gas separation factor (selectively) and high gas permeability. Gas separation membranes also are preferably fabricated from polymers which are easily processed. The problem is finding membrane materials which possess all the desired characteristics. Polymers possessing high separation factors generally have low gas permeabilities, while those polymers possessing high gas permeabilities generally have low separation factors. In the past, a choice between a high gas separation factor and a high gas permeability has been unavoidably necessary. Furthermore, some of the polymeric membrane materials previously used suffer from the disadvantage of poor performance under high operating temperatures and pressures. However, those polymeric membrane materials capable of operating at high temperatures and pressures are typically difficult to fabricate into membranes. A membrane capable of separating gas mixtures which possesses high selectivity, high gas permeability, ability to operate under extreme conditions of temperature and pressure, and ease of fabrication is needed.